The present invention relates to the calibration of pressure-sensitive hydrophones and more particularly to an improved system for calibrating hydrophones down to very low frequencies.
In the field of underwater acoustic signal detection, pressure-sensitive hydrophones, individually and oftentimes collectively in linear arrays, are relied upon to produce meaningful electrical signals in response to waterborne sonic waves. To accurately interpret the acoustic information contained in these electrical signals, it is necessary to calibrate hydrophones in a working environment so that a baseline frequency response is provided for a particular hydrophone over a complete acoustic frequency range. Of particular value is the ability to calibrate hydrophone systems in the lower portion of the infrasonic range, generally less than 10 Hz, because of potential inaccuracies produced by extrapolating the results of higher audio frequency testing. In addition, very low frequency hydrophone systems are being developed that are in need of an effective and reliable means for measuring their sensitivity.
Pressure hydrophones have been typically calibrated in tanks or small lakes with known signals from an acoustic projector. These projectors generally have physical parameters that limit their outputs to frequencies at or above 10 Hz, and only extremely large, awkward devices have been able to produce acoustic pressure signals down to about 5 Hz. As a result, calibrations may be performed acoustically down to 5-10 Hz, but extrapolation is required in order to determine lower frequency responses.